A cam drive gear for an engine is disclosed in Japanese Patent No. 3309700 in which a primary drive gear provided on a crankshaft is meshed with an idle gear rotatably borne on an idle shaft provided on an engine main body while having an axis parallel to the crankshaft, and drive sprockets are rotated together with the idle gear.
The primary drive gear provided on the crankshaft is formed with a comparatively large diameter so as to transmit rotational torque of the engine to the transmission side. In the cam drive gear disclosed in Japanese Patent No. 3309700, the primary drive gear with the comparatively large diameter is meshed with the idle gear, so that it is necessary for the distance between the axes of the crankshaft and the idle shaft rotatably supporting the idle gear to be set comparatively large, leading to an increase in the size of the engine.
In addition, a valve-operating system drive gear for an engine is disclosed in Japanese Patent Publication No. Hei 4-75364. The valve-operating system drive gear includes a driven sprocket provided on a camshaft rotatably borne on a cylinder head, a drive sprocket operated in conjunction with the rotation of a crankshaft, an endless form cam chain wrapped around the drive sprocket and the driven sprocket, and a chain guide member making sliding contact with the outer periphery of the cam chain. The chain guide member makes sliding contact with the outer periphery of the cam chain so as to restrain the cam chain from chattering between the drive sprocket and the driven sprocket.
In the valve-operating system drive gear disclosed in Japanese Patent Publication No. Hei 4-75364, both end portions of the chain guide member are curved so as to be spaced from the drive sprocket and the driven sprocket. However, the cam chain may chatter at its portions wrapped around the sprockets due to, for example, elongation of the cam chain or wear of the cam chain or of the drive sprocket and the driven sprocket. Such a chattering of the cam chain may occur at the portion of the cam chain wrapped around the drive sprocket due to the weight of the cam chain itself.